Tuesday, January 28, 2014

But as he spoke, Brown hit a darker note. Last week, amid the driest
year for the state since record-keeping began in the 1840s, Brown
declared a drought emergency for California, and in his speech he warned of harder times ahead:

Among all our uncertainties, weather is one of the most
basic. We can’t control it. We can only live with it, and now we have to
live with a very serious drought of uncertain duration…We do not know
how much our current problem derives from the build-up of heat-trapping
gasses, but we can take this drought as a stark warning of things to
come.

(MORE: Can GM Crops Bust the Drought?)
Californians need to be ready, because if some scientists are right,
this drought could be worse than anything the state has experienced in
centuries. B. Lynn Ingram, a paleoclimatologist at the University of
California, Berkeley, has looked at rings of old trees in the state,
which helps scientists gauge precipitation levels going back hundreds of
years. (Wide tree rings indicate years of substantial growth and
therefore healthy rainfall, while narrow rings indicate years of little
growth and very dry weather.) She believes
that California hasn’t been this dry since 1580, around the time the
English privateer Sir Francis Drake first visited the state’s coast:

If you go back thousands of years, you see that droughts
can go on for years if not decades, and there were some dry periods that
lasted over a century, like during the Medieval period and the middle
Holocene [the current geological epoch, which began about 11,000 years
ago]. The 20th century was unusually mild here, in the sense that the
droughts weren’t as severe as in the past. It was a wetter century, and a
lot of our development has been based on that.

Ingram is referring to paleoclimatic evidence that California, and
much of the American Southwest, has a history of mega-droughts that
could last for decades and even centuries. Scientists like Richard
Seager of Columbia University’s Lamont-Dohery Earth Observatory have used
tree-ring data to show that the Plains and the Southwest experienced
multi-decadal droughts between 800 A.D. and 1500 A.D. Today dead tree
stumps—carbon-dated to the Medieval period—can be seen in river valley
bottoms in the Sierra Nevada mountains, and underwater in places like
California’s Mono Lake, signs that these bodies of water were once
completely dry. Other researchers have looked at the remains of bison
bones found in archaeological sites, and have deduced that a millennium
ago, the bison were far less numerous than they were several centuries
later, when they blanketed the Plains—another sign of how arid the West
once was. The indigenous Anasazi people of the Southwest built great
cliff cities that can still be seen in places like Mesa Verde—yet their civilization collapsed, quite possibly because they couldn’t endure the mega-droughts.